Oil-impregnated sintered bearings are a kind of plain bearings comprising porous bodies made by pressing and heating metallic powders typically powders of copper, iron, tin, zinc or the like, which are impregnated with a lubricating oil, and are used in a self-oiling state. In spite of the low cost, the oil-impregnated sintered bearings are of relatively low friction and can be processed with a high precision in a self-lubricating system, and thus have been widely used as motor bearings in every parts of automobile electrical components, audiovisual components, business machinery, household electrical appliances, and driving components for computer auxiliary memory devices.
Recent enhancement of performances of these machinery and components has also been imposing very severe requirements for performances to the oil-impregnated sintered bearings such as down-sizing, higher speed, and lower electric current level and lower electric power consumption. To meet these requirements, it is not only important for the bearing manufacturers to further study the bearing materials, etc. on one hand, but also important to study further improvement of lubricating oil, because the characteristics or life of motors are largely dependent on performances of a few milligrams of lubricating oil impregnated and retained in the bearings on the other hand.
Characteristics required for the lubricating oil for the oil-impregnated sintered bearings include, for example, good compatibility with bearing materials (no generation of corrosion, sludges, etc.), for example, in the case of automobile application, availability in a wide temperature range such as −40° to +180° C. (low evaporation loss and good oxidation stability at high temperature, and no deterioration of flowability at low temperature), good anti-rust property, and substantially no adverse effect on resin parts, low friction coefficient and good wear resistance to meet the recent requirements such as the down-sizing, lower electric current level and longer life.
Perfluoropolyether oil has a distinguished heat stability, low volatility and chemical resistance, substantially no adverse effect on resin parts, and a distinguished temperature-viscosity characteristic, thus enabling application as a suitable lubricating oil for the oil-impregnated sintered bearing. In this connection, the following Patent Literature 1 proposes oil-impregnated sintered bearings impregnated with a perfluoropolyether oil to attain the desired stability of lubricating characteristics and prolonged bearing life on the basis of the low volatility of perfluoropolyether oil.
Patent Literature 1: JP-A-5-240251
However, the perfluoropolyether oil has such disadvantages as high wettability and diffusivity due to the low surface tension. Thus, its high leakability and scatterability have often caused problems. The oil-impregnated sintered bearing has been endowed with a longer life by impregnating and holding a lubricant in the pores of the bearing, thereby slowly oozing the lubricant to the sliding surface, but the above-mentioned properties of perfluoropolyether oil, i.e. high leakability and scatterability cause the oil to ooze out from the bearing more rapidly than as required, resulting in earlier consumption of the oil as held in the bearing, thereby shortening the life of bearing. Furthermore, the oil oozed out from the bearing will inevitably pollute the surrounding circumstances other than where desired.
The following Patent Literature 2, on the other hand, proposes a porous bearing filled with a grease containing at least 0.1% of a thickener. However, when the fiber width or particle sizes of the thickener are larger than pore sizes of the oil-impregnated sintered bearing, the thickener itself can hardly enter into the pores of the oil-impregnated sintered bearing, or rather will promote pollution of bearing circumstances.
Patent Literature 2: JP-A-63-195416
Perfluoropolyether oil also has a poor compatibility with metals, as compared with ordinary synthetic oil such as ester oil or synthetic hydrocarbon oil, and thus cannot form a satisfactory oil film for lubrication under severe conditions, so that it is less satisfactory than the ordinary synthetic oils in respect to wear resistance and friction resistance. The anti-rust property of perfluoropolyether oil is much more deteriorated thereby than that of the ordinary synthetic oil. When an additive is added thereto to overcome the above-mentioned problems, the perfluoropolyether oil has, however, a poor compatibility with non-fluorine-based additives so far used in most of the ordinary synthetic oils. Even if the perfluoropolyether oil is mixed with so far proposed various fluorine-based additives, on the other hand, it is hard to attain satisfactory wear resistance, low friction coefficient characteristics and anti-rust property.
The present applicant has so far proposed a lubricant composition for oil-impregnated sintered bearings, which comprises a base oil consisting of specific two kinds of polyfluoropolyether oils, and fluororesin powders having primary particle sizes of not more than 1 μm. The lubricant composition, when used in oil-impregnated sintered bearings, can improve the wear resistance, low friction coefficient, scattering-leaking resistance, and anti-rust property. Recently, requirements for lower torque have been imposed, while keeping the above-mentioned performances at the satisfactory level. Thus, further improvement of wear resistance and low friction coefficient characteristics has been now a new task.
Patent Literature 3: JP-A-2003-147380